Acoustic feedback and external noise in hearing aids are problems, which have been compensated in a number of ways in the prior art.
In regards to acoustical feedback several known methods are used for reducing the negative effects introduced by acoustic feedback in a hearing aid, this includes notch filtering, frequency compression, modification of the phase response, and feedback cancellation, such as disclosed in M. Sc. Thesis entitled “Digital suppression of acoustic feedback in hearing aids” written by Best L. C. and written for the Department of Electrical Engineering, University of Wyoming, 1985.
Best's thesis describes a method using a least-mean-square (LMS) filter technique for estimating external acoustic feedback, which estimate is used for feedback cancellation in a hearing aid. The estimate is subtracted from the input signal thus removing the acoustic feedback.
Further, in European patent application no.: EP 1 216 598 several prior art systems attempting to eliminate unstable feedback in hearing aids are presented and their disadvantages considered. The European patent application therefore suggests a system for overcoming these disadvantages, which system comprises a signal processor processing an audio input signal including a feedback component associated with an acoustic feedback path, and comprises a detector detecting the feedback component and issuing a feedback indicator parameter signal to a probe generator generating a narrowband probed signal to probe the acoustic feedback path. The system further comprises a feedback-inhibiting filter controlled by a filter adjuster in accordance with the feedback indicator parameter signal received by the detector. Hence the system utilises a high signal-to-noise sub-audible probe signal to establish the extent of the acoustic feedback of the system and adjusts the feedback-inhibiting filter accordingly. Even though this system reduces the effects of acoustic feedback, filtering of the incoming signal to remove acoustic feedback distorts the acoustic sounds to be presented to the user of the hearing aid, since the feedback-inhibiting filter removes some of the original signal in the process, which is not restored. In addition, this feedback cancellation technique relies on a high degree of accuracy of the estimation of the potentially dynamic, external acoustic feedback. Erroneous estimations of the acoustic feedback introduce audible distortions to the original input signal due to the subtraction.
Further, Ph. D. thesis entitled “Compensation for hearing loss and cancellation of acoustic feedback in digital hearing aids” written by Hellgren, J and written for Linköping Studies in Science and Technology reveals feedback cancellation techniques using the input signal as well as the output signals to estimate the acoustic feedback path are sensitive to signals that are correlated between the input. For example music with tonal inputs may cause the feedback cancellation system to try to cancel the tonal parts of the music thus degrading sound quality for the user of a hearing aid.
In light of above reference prior art there is a need for feedback cancellation systems and methods for removing more of the acoustic feedback, ideally completely removing the acoustic feedback, which systems and methods avoid the introduction of audible distortions.
In regards to noise reduction, “Noise reduction in hearing aids: What works and why” and article written by Donald Schum and published in News from Oticon, April 2003, provides a review of state of the art noise reduction techniques in hearing devices. Several of the digital signal processor (DSP) based instruments on the market implement variations of modulation detection for classifying the input as either speech or noise. According to this scheme, the on-going amplitude modulations of the input signal are monitored. Speech in quiet is known to have relatively deep (15 dB or greater) modulations at a rate between approximately 3 to 10 Hz. This modulation pattern reflects the syllabic structure of speech: 3 to 6 syllables per second. In contrast, certain environmental sounds tend to be more stable in terms of on-going amplitude. It is unusual for a non-speech noise source to have a modulation rate and depth similar to that of speech.
As implemented in hearing aids, the input is divided into multiple channels. The modulation behaviour is monitored in each channel. If the modulation rate and depth is similar to speech, then that channel is passed without gain reduction. If the modulation behaviour in the channel is more stable, it is assumed that that channel is dominated by steady state noise and gain reductions are applied. However, this may introduce a distortion of the original speech signal in presence of noise, since the noise-dominated channels/bands are attenuated if they are classified as noisy. Therefore, there is a need for systems and methods that reduces noise without attenuating the speech part in the channels that has been classified as noisy.